The invention disclosed and claimed herein pertains to the field of communications engineering. More particularly, the invention pertains to the fields of transmission system design and modulation theory. In still greater particularity, this invention pertains to systems in which pulsed signals are transmitted through a fiber optic transmission apparatus. The field of the invention includes the field of modulating the repetition frequency of pulses of the above type with the amplitude of a video signal.
Video signals comprise the set of communication signals whose power spectra reveal a wide frequency range containing no DC component. Most of the video signal power is concentrated in the lower frequencies; however, significant information value is contained in the higher frequencies. Television signals and audio signals, for example, fall into this field.
A need exists for long-distance, non-broadcast transmission of video information. For example, distribution of closed-circuit television signals, area surveillance, observation of deep-sea phenomena, and control of industrial production lines are all applications requiring inexpensive, reliable transmission of video signals over long distances. The indicium of quality applying to a long-distance video transmission system is the signal-to-noise ratio (SNR) of the transmitted signal. A high SNR results in realization of lengthened transmission distance.
Long distance transmission of video signals through conduction cables has been limited in the past by the characteristic gain-bandwidth products of typical metallic conductors. Utilization of a high quality, wideband, low loss optical fiber transmission channel has been shown to significantly increase the distance over which a video signal can be transmitted. With high power, high speed injection laser diodes, and high gain avalanche photodetectors included in fiber optic system design, transmission distance can be further extended.
In a fiber optic transmission system, transmission by intensity modulation (IM), where the amplitude of the transmitted video signal is represented by the intensity of the conducted optical signal, is inefficient and minimally effective. The narrowband IM signal wastes available transmission bandwidth. The modulation index is minimized by the limited linearity of available optical sources; the continuous IM signal increases photodetector shot noise; both factors reduce realizable SNR. As a consequence of the limited effectiveness of IM, frequency modulation (FM) has received a great deal of attention as a modulation technique particularly suited to fiber optic transmission.
Frequency modulation, wherein a change in amplitude of a modulating signal is represented as a change in frequency of a carrier signal, has been used in fiber optic transmission to take greater advantage of available transmission bandwidth and to enhance the SNR of the transmitted signal through the well-known processing gain associated with FM. Because FM is a nonlinear technique, its application is not limited by nonlinearities associated with optical transmitters. There are two principal disadvantages in applying FM to fiber optic transmission which limit the realizable SNR. Classical FM techniques, which are based upon modulation of a continuous signal with a 50 percent duty factor, result in a significant level of average transmitted power which can result in the stimulation of shot noise in the photodetector. The 50 percent duty factor of the FM carrier also reduces the peak power amplitude obtainable from optical power sources whose lifetimes are directly related to average power dissipation.
Transmission techniques based upon the modulation of widely spaced, narrow pulses are known to benefit the SNR performance of fiber optic transmission systems. The low duty cycle of pulse modulation (PM) allows an optical transmitter to provide high output power peaks at no cost in life expectancy. In addition, output power peaks can be averaged over the low duty cycle to reduce average signal power incident on the photodetector which results in a diminution of background shot noise.
A technique of PM is exemplified in U.S. Pat. No. 3,899,429, 1975, issued to Y. Ueno, et al., which discloses the use of a pulse-position modulation (PPM) technique in which the time positions of transmitted pulses relative to each other are modulated by a signal which is the algebraic summation of an information signal and a ramp signal. Such a technique takes advantage of the superior operation of fiber optic transmission systems under pulsed conditions; however, it is known that the signal-to-noise performance of PPM does not match that of FM. (Carlson, A. B., Communication Systems: An Introduction to Signals and Noise in Electrical Communication, McGraw-Hill, Inc., 1968, pp. 299-302).
It has been suggested in the literature that a pulse-frequency modulation (PFM) approach might combine the desirable aspects of FM and PM when employed in a fiber optic transmission system. (Timmermann, C. C., "Signal-to-Noise Ratio of a Video Signal Transmitted by a Fiber-Optic System Using Pulse-Frequency Modulation," IEEE Transactions on Broadcasting, Vol. BC-23, No. 1, March 1977, pp. 12-17). However, the article lacks the analysis and synthesis of a PFM fiber optic video transmission system which would enable one skilled in the art to design and fabricate one. A design method for such a system is described in a recent publication. (Cowen, S. J., Proceedings IEEE Oceans 1979 Conference, "Fiber Optic Video Transmission System Employing Pulse Frequency Modulation," CH 1478-7/79/0000-0253, September 1979 pp. 253-259).
Pulse-frequency modulation is a method in which the amplitude of a modulating signal is represented by the repetition frequency of a stream of transmitted pulses having constant widths. As such, the spectrum of PFM is composed of a series of harmonically-related, phase-locked FM spectra having theoretically infinite bandwidths. (Panter, P. F., Modulation, Noise, and Spectral Analysis, McGraw-Hill, N.Y. 1965, pp. 506, 536). The envelope of the amplitude spectrum of the unmodulated PFM carrier displays the familiar sin (X)/X form, having a zero order component with a sideband. This suggests the use of low-pass filtering to recover the envelope of a modulating signal.
The prior art reveals no concept for a fiber optic video transmission system employing pulse frequency modulation which provides a simple, efficient system design intended to maximize SNR of the transmitted signal by fully utilizing the gain-bandwidth product of an optical fiber.